This Phase II proposal seeks support for completing implementation of effective fragment potential (EFP) method [J. Phys. Chem. A, v.105 p.293 (2001)] in the Q-Chem electronic structure program. The EFP approach enables one to treat large systems with localized interactions by separating them into a small "important" part (e.g., reaction center) treated quantum mechanically (QM), and the environment, which is further subdivided into the so-called effective fragments (EFs). Conceptually, the EFP method is similar to the popular QM/MM (molecular mechanics) scheme;however, it replaces empirical MM force fields by rigorous interactions derived from QM calculations of individual fragments. Once the necessary parameters of EFs are pre-computed and stored in an auxiliary database, the cost of an EF calculation is very similar to that of a QM/MM one. During Phase I, we completed most of the steps necessary for pre-computing EF parameters and energy calculation. During Phase II, we propose to complete energy calculation, as well as implement analytic gradient calculation, which is crucial tool for computational research. The full implementation of the EFP method in Q-Chem will enable the researchers to apply advanced QM methods (e.g., equation-of-motion and coupled-cluster methods) to study opens-shell and electronically excited centers in biological molecules, solutions, and materials. PUBLIC HEALTH RELEVANCE: Quantum modeling is the most accurate and versatile among different molecular simulation methods of biological systems. In this SBIR Phase II application, we propose to implement and develop a method that will enable accurate quantum mechanical modeling for large systems. The resulting program will significantly increase researchers'quality of work will extend the application scope of quantum methods for the simulations of biological systems.